1. Field of the Invention
The present invention relates to a fuel cell system including an internal manifold type fuel cell formed by stacking an electrolyte electrode assembly and separators. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. At least reactant gas supply passages and reactant gas discharge passages extend through the fuel cell in the stacking direction.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. In use, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In this case, some of the fuel gas supplied to the anode is not consumed in the power generation, and discharged from the fuel cell. Since the unconsumed fuel gas is discharged wastefully from the fuel cell, good fuel economy may not be achieved. In order to improve the fuel economy, in a proposed structure, the unconsumed fuel gas discharged from the fuel cell is mixed with the fresh fuel gas, and supplied again to the anode.
The operating temperature of the solid polymer fuel cell is relatively low (100° C. or less). Therefore, after the water flows into the fuel cell stack, the water which has not been absorbed to the electrolyte membrane or the water produced in the power generation is cooled in the reactant gas flow field (the oxygen-containing gas flow field and/or the fuel gas flow field) of the fuel cell stack or the reactant gas flow passage (the oxygen-containing gas passage and/or the fuel gas passage) as a manifold extending through the fuel cell stack in the sacking direction, and connected to the reactant gas flow field. The water is likely to be retained in the liquid state.
However, in the presence of water in the reactant gas flow field or the reactant gas passage of the fuel cell stack, it is difficult to sufficiently supply the oxygen-containing gas or the fuel gas to each of the unit cells. Therefore, the fuel gas and the oxygen-containing gas as reactant gases are not diffused to the electrode catalyst layers efficiently. Thus, the power generation performance is degraded significantly.
In an attempt to address the problem, Japanese Laid-Open Patent Publication No. 7-192743 discloses a fuel cell system 1 as shown in FIG. 7. The fuel cell system 1 includes a solid oxide fuel cell 2. The fuel cell 2 is connected to a fuel system 3 for supplying a hydrogen gas to the fuel cell 2, an air system 4 for supplying the air as an oxygen-containing gas to the fuel cell 2, and a coolant water circulation system 5 for supplying the coolant water to the fuel cell 2 by circulation.
According to the disclosure, the fuel system 3 supplies the hydrogen gas in a hydrogen absorption storage alloy tank 6 to the fuel cell 2 through a hydrogen supply pipe 7. The excessive hydrogen gas discharged from the fuel cell 2 is supplied to a gas liquid separator 8, and the water mixed in the hydrogen gas is stored as condensed water. The hydrogen gas in the gas liquid separator 8 is circulated to the hydrogen supply pipe 7 by a pump 9. Thus, the water in the excessive hydrogen gas discharged from the fuel cell 2 is stored as condensed water in the gas liquid separator 8. After water is removed, the hydrogen gas is supplied again as the fuel gas to the fuel cell 2.
However, in Japanese Laid-Open Patent Publication No. 7-192743, it is likely that the excessive hydrogen gas is mixed into a large amount of water produced in the power generation, and then, flows into the gas liquid separator 8. Therefore, a considerably large amount of hydrogen gas is mixed into the condensed water in the gas liquid separator 8. The consumption amount of hydrogen gas is large, and the desired fuel economy cannot be achieved.
In the case where a large amount of hydrogen gas is mixed into the condensed water stored in the gas liquid separator 8, at the time of diluting the condensed water, and discharging the diluted water to the outside, it is necessary to increase the amount of the dilution gas supplied by the air system 5. Thus, the energy consumption of devices such as a pump or a supercharger for supplying the oxygen-containing gas is increased, and the desired fuel economy cannot be achieved.
As described above, in the structure of supplying the fuel gas to the fuel cell by circulation, the water (produced in the power generation) discharged from the fuel cell is supplied again to the fuel cell. Therefore, flooding may occur at the anode. Further, when nitrogen in the air supplied to the cathode passes through the solid polymer electrolyte membrane, the nitrogen may be mixed into the fuel gas, and the nitrogen concentration may be increased undesirably.
Therefore, a process of purging the fuel gas containing water and nitrogen from the fuel gas circulation passage is performed. In this regard, for example, a hydrogen purging control apparatus is disclosed in Japanese Laid-Open Patent Publication No. 2004-55287. As shown in FIG. 8, a fuel cell system including the hydrogen purging control apparatus has a fuel cell 1a. The air pressurized at a predetermined pressure by a compressor 2a is supplied to the cathode (not shown) through an air supply channel 3a. Further, a hydrogen gas is supplied from a hydrogen tank 4a to the anode (not shown) of the fuel cell 1a through a hydrogen gas supply channel 5a. 
The hydrogen gas which has not been consumed in the reaction in the fuel cell 1a is discharged to a hydrogen gas circulation channel 6a together with the water produced in the power generation, and flows into the hydrogen gas supply channel 5a by operation of an ejector 7a. A purging hydrogen diluter 9a is connected to the hydrogen gas circulation channel 6a through a purging valve 8a. The air discharged from the fuel cell 1a flows into the purging hydrogen diluter 9a through an air discharge channel 3b together with the water produced.
Therefore, at the time of purging, the purging valve 8a is opened such that the hydrogen gas discharged from the fuel cell 1a flows into the purging hydrogen diluter 9a together with the water produced in the power generation. Since the air discharged from the fuel cell 1a and the water produced in the power generation are supplied into the purging hydrogen diluter 9a, the hydrogen gas is mixed with the discharged air, and diluted. After the hydrogen concentration is adjusted to a predetermined concentration, the hydrogen gas is discharged to the outside.
In Japanese Laid-Open Patent Publication No. 2004-55287, the air supplied to the fuel cell 1a is discharged to the air discharge channel 3b together with the water produced in the reaction at the cathode. Therefore, by the change in the amount of the produced water, the air pressure in the fuel cell 1a changes easily. Thus, it is difficult to control the flow rate of the air supplied to the fuel cell 1a. 